Patients experiencing cerebral ischemia often suffer from disabilities ranging from transient neurological deficit to irreversible damage (stroke) or death. Cerebral ischemia, i.e., reduction or cessation of blood flow to the central nervous system, can be characterized as either global or focal. Global cerebral ischemia refers to reduction of blood flow within the cerebral vasculature resulting from systemic circulatory failure caused by, e.g., shock, cardiac failure, or cardiac arrest. Shock is the state in which failure of the circulatory system to maintain adequate cellular perfusion results in reduction of oxygen and nutrients to tissues. Within minutes of circulatory failure, tissues become ischemic, particularly in the heart and brain.
The two common forms of shock are cardiogenic shock, which results from severe depression of cardiac performance, and hemorrhagic shock, which results from trauma. The most frequent cause of cardiogenic shock is myocardial infarction with loss of substantial muscle mass. Pump failure can also result from acute myocarditis or from depression of myocardial contractility following cardiac arrest or prolonged cardiopulmonary bypass. Mechanical abnormalities, such as severe valvular stenosis, massive aortic or mitral regurgitation, acutely acquired ventricular septal defects, can also cause cardiogenic shock by reducing cardiac output. Additional causes of cardiogenic shock include cardiac arrhythmia, such as ventricular fibrillation. Hemorrhagic shock is typically the result of penetrating injuries caused by, for example, traffic accidents and gunshot wounds. In this case, cardiac function is unimpaired and the cause of shock is circulatory blood loss.
Treatment of global cerebral ischemia involves treating the source of the systemic circulatory failure and ensuring adequate perfusion to the central nervous system. For example, treatment of cardiogenic shock due to prolonged cardiopulmonary bypass consists of cardiovascular support with the combination of inotropic agents such as dopamine, dobutamine, and intra-aortic balloon counterpulsation. Treatment of hemorrhagic shock consists of volume replacement and hemostasis. When these measures fail, supracoeliac aortic clamping is used. Vasoconstrictors, such as norepinephrine, are also administered systemically to maintain systolic blood pressure (ideally above 80 mmHg). Unfortunately, these agents produce pressure at the expense of flow, particularly blood flow to small vessels such as the renal arteries. The use of the vasoconstrictors is, therefore, associated with significant side effects, such as acute renal failure, congestive heart failure, and cardiac arrhythmias.
Focal cerebral ischemia refers to cessation or reduction of blood flow within the cerebral vasculature resulting from a partial or complete occlusion in the intracranial or extracranial cerebral arteries. Such occlusion typically results in stroke, a syndrome characterized by the acute onset of a neurological deficit that persists for at least 24 hours, reflecting focal involvement of the central nervous system and is the result of a disturbance of the cerebral circulation. Other causes of focal cerebral ischemia include vasospasm due to subarachnoid hemorrhage or iatrogenic intervention.
Traditionally, emergent management of acute ischemic stroke consists of mainly general supportive care, e.g. hydration, monitoring neurological status, blood pressure control, and/or anti-platelet or anti-coagulation therapy. Heparin has been administered to stroke patients with limited and inconsistent effectiveness. In some circumstances, the ischemia resolves itself over a period of time because some thrombi get absorbed into the circulation, or fragment and travel distally over a period of a few days. In June 1996, the Food and Drug Administration approved the use of tissue plasminogen activator (t-PA) or Activase®, for treating acute stroke. However, treatment with systemic t-PA is associated with increased risk of intracerebral hemorrhage and other hemorrhagic complications. Vasospasm may be partially responsive to vasodilating agents. The newly developing field of neurovascular surgery, which involves placing minimally invasive devices within the carotid arteries to physically remove the offending lesion, may provide a therapeutic option for these patients in the future, although this kind of manipulation may lead to vasospasm itself. Iatrogenic vasospasm and vasospasm caused by subarachnoid hemorrhage may respond to treatment with aortic constriction.
In both global and focal ischemia, patients develop neurologic deficits due to the reduction in cerebral blood flow. One treatment may include the use of devices to increase blood flow to the cerebral vasculature as the sole therapy. Alternatively, treatments include measures to increase blood flow to the cerebral vasculature to maintain viability of neural tissue, thereby increasing the length of time available for any adjunct interventional treatment and minimizing neurologic deficit while waiting for resolution of the ischemia. Augmenting blood flow to the cerebral vasculature is not only useful in treating occlusive or vasospastic cerebral ischemia, but may also be useful during interventional procedures, such as carotid angioplasty, stenting or endarterectomy, which might otherwise result in focal cerebral ischemia, and also cardiac procedures which may result in cerebral ischemia, such as cardiac catheterization, electrophysiologic studies, and angioplasty.
New devices and methods are thus needed for augmentation of cerebral blood flow in treating patients with either global or focal ischemia caused by reduced perfusion, thereby minimizing neurologic deficits.